Sunglasses last as long as you take care of them. For many years, the only time that sunglasses came off my face, they went into a hard case. That's the best way to take care of them.
I wear prescription sunglasses, so I alternate regular glasses and sunglasses. When I take the sunglasses off, they go into an Oakley microfiber bag that I handle carefully or keep in a specific pocket in my briefcase. They don't go into a pocket loose, never onto a car's dash unless they're in the protective microfiber bag, etc. They don't get scratched, they don't get sat on. I'm careful about that when they cost $300 a pair.
My sunglasses last 5-8 years per pair. My current pair is American Optical aviators, and they're on their second set of prescription lenses so I'm sure they're 4+ years old.
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Take care of them and they'll last. Treat them roughly and they'll die quickly. Cost is irrelevant because it doesn't increase their ability to withstand rough handling.